1. Technical Field
Embodiments discussed herein generally relate to an image processing technology.
2. Related Art
Observation of fine patterns is required in various fields, such as semiconductor manufacturing and medical fields. For example, in the semiconductor manufacturing field, semiconductor devices and the like have been integrated or decreased in size to increasingly higher degrees, and improvements in performance of charged particle beam apparatuses for observing and measuring such samples are desired. For example, in a scanning electron microscope (hereafter referred to as “SEM”), which is a type of charged particle beam apparatus, high resolution and reproducibility are required. Also, in order to achieve high resolution and reproducibility, contrast and brightness of an image for sample observation, measurement, or inspection by the SEM need to be properly adjusted, and there is a room for improvement in relevant image processing technology.
Signal electrons that are emitted by excitation of atoms at the surface of an observed sample upon irradiation with an electron beam and that have low energy are referred to as “secondary electrons”. When an edge portion of a sample pattern with convex-concave, such as a semiconductor circuit pattern, is irradiated with an electron beam, the amount of secondary electron generated is increased by an edge effect, resulting in an image with an irregularity-dependent contrast.
Electrons that are re-emitted from the sample surface in the process of scattering of the electron beam within the sample are referred to as “back-scattered electrons”. The back-scattered electrons are signal electrons with high energy. The back-scattered electrons provide an image with a contrast that depends on the sample (material) composition, rather than the convex-concave of the sample.
In recent years, due to the increasingly complex processes for semiconductors, magnetic heads and the like, measuring of low-step samples with no convex-concave on the observed sample surface is required. In this case, the amount of secondary electron signal that is detected is decreased because of the low step. Thus, improved image quality and length measurement accuracy can be obtained by detecting the back-scattered electrons instead of, or together with, the secondary electrons, and by enhancing the edge or contrast information by composing a signal or image on the basis of the back-scattered electrons. Detection through the back-scattered electrons is also possible even when a barrier for the secondary electrons is formed by charging of the surface of a sample, such as a sample containing insulator material, on the order of several to several dozens of volts due to the irradiation by the electron beam during SEM observation. Because the contrast of a back-scattered electron image is determined depending on the atomic number of the material, a sharp image can be obtained from a pattern of materials with a large atomic number difference.
JP Patent Publication (Kokai) No. 2004-208044 A discloses a contrast and brightness adjustment method for obtaining an appropriate gradation. The method involves acquiring a bright image and a dark image with respect to an original image acquired with a predetermined contrast or brightness, extracting a dark portion from the bright image, extracting a bright portion from the dark image, and then composing the extracted dark portion image and bright portion image.
However, according to the image processing technology using the contrast adjustment method disclosed in the above publication, when a back-scattered electron image of a pattern containing materials with close atomic numbers is acquired, the image has a small contrast difference.
As one of the ways to increase contrast partly so as to provide an image contrast difference, intermediate luminance emphasis is known. This technology, however, has the problem that when the luminance of various materials is expressed in a histogram, waveforms in the histogram are overlapped between the materials, so that the range for luminance emphasis cannot be properly set.
Thus, it is difficult to obtain high resolution according to the related art including the above technology. When the resolution is low during image observation, length measurement reproducibility, for example, is decreased.